An ion trap is an apparatus used to confine or isolate a charged particle, such as an electron. One class of such apparatus is known as Penning traps. In general a Penning trap uses a magnetic field and an electrostatic field together to trap charged particles. The magnetic field causes the charged particles to perform a rotational movement with the direction of the magnetic field being the axis of the rotation. This effectively confines the particles to a plane normal to the direction of the magnetic field. The electrostatic field is arranged to confine the charged particles at a location along the direction of the magnetic field, by providing a potential well for the particle at the desired location.
In order for a Penning trap to confine a charged particle effectively and to be useful for performing measurements on the trapped particle, it is important both for the magnetic field to be spatially homogeneous and for the electrostatic field to be hyperboloid at the location the charged particle is to be trapped. This places constraints in the design of a conventional Penning trap and its variants, generally making them complex and expensive.
In general, the required electrostatic potential well is created by a set of metallic electrodes with appropriate static voltages applied to them. Conventional Penning traps are fabricated with three-dimensional (3D) electrodes. The first Penning trap used electrodes with the shapes of hyperboloids of revolution. This guarantees that the electrostatic potential well nearly follows the ideal shape of a harmonic potential well. Penning traps with the electrodes of that shape are called “hyperbolic Penning traps”. In 1983 a Penning trap with electrodes with the shapes of cylinders was introduced. This is now a common type of conventional Penning trap and it is known as the “cylindrical Penning trap”.
Conventional Penning traps employ a solenoid, usually superconducting, to create the required magnetic field. Solenoids are big, unscalable and very expensive structures. In order to achieve the required spatial homogeneity solenoid systems include, besides the main coil, additional shim-coils with carefully chosen shim-currents. The fields created by the shim-coils cancel inhomogeneities of the bulk magnetic field in a big volume enclosing the position where the charged particles are trapped. The time stability of the magnetic field is achieved with passive coils. These damp any fluctuations of the field caused by external magnetic noise of whatever origin. Normal conducting solenoids are too unstable for high precision mass spectrometry and for quantum computation applications with trapped electrons. Superconducting solenoids are typically room-size devices. The magnetic field in the trapping region, i.e. the region in the immediate vicinity of the trapped charged particles, cannot be isolated within a superconducting shield-box. The latter is the most effective protection against external magnetic noise. The temperature of the room-size superconducting solenoid system cannot be regulated with a stability and accuracy below the level of 1 K. This is due to the big size of the solenoid, which requires stabilizing the temperature of the room where the solenoid with the ion trap is located.